BAKED CONTACT TEST DEMONSTRATES PRIMARY NATURE OF DOMINANT (N1) MAGNETIZATION OF NIPISSING INTRUSIONS IN SOUTHERN PROVINCE, CANADIAN SHIELD

Nipissing diabase sills, cone sheets and dykes define a major episode of igneous intrusion in the Southern Province of the Canadian Shield at about 2200 Ma. The dominant (N1) remanence of these intrusions has been established in several palaeomagnetic studies and, hence, is a key tie point for the North American apparent polar wander path. Yet the age of this magnetisation remains very controversial. Positive baked contact tests reported for the N1 remanence have often been dismissed as inconclusive and the N1 palaeopole attributed to overprinting during the Hudsonian Orogeny (1900-1800 Ma) due to its position on a widely accepted polar wander path for North America. Previous baked contact studies utilise sediments of the Huronian Supergroup as host rock. The remanence of these sediments is often difficult to interpret. In this study a positive baked contact test is described for an N1 Nipissing sill or cone sheet using stably magnetized mafic dykes of the Matachewan swarm as host. Thus, the N1 remanence is primary and no longer supports the time calibration of polar wander paths which assume that it is a 1900-1800 Ma overprint.